The default limits noted on the page have remained outdated after merge
request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/27574
overrode the previous limit defaults from disabled to
specific values, for a certain subset of limits.

In the existing form the documentation claims no limits on CI/CD
subscriptions and CI Pipeline Schedules created under a project, but
this has been incorrect since the introduction, as the `default` plan
carries non-zero, positive defaults for such instance limits.

Also added a note on self-managed instance default limits for
group and project webhooks.